The present invention relates to a developing apparatus used in an electrophotographic image forming apparatus. The developing apparatus is so arranged that an electrostatic latent image formed on an image supporting body is visualized by using developer. The present invention especially relates to a developing apparatus so arranged that developer is transported to a development position on an image supporting body by using a progressive wave electric field.
Among developing apparatuses used in image forming apparatuses, such as copying apparatuses and printers, that perform electrophotographic image forming, some developing apparatuses are so arranged that a developer supporting body, which supplies developer to an image supporting body, is positioned so as not to contact a surface of the image supporting body. Well-known methods employed in such developing apparatuses are a powder cloud method, a jumping method, and an electric field curtain (progressive wave electric field) method. The electric field curtain method is employed in a developing apparatus disclosed in Japanese Publication for Unexamined Patent Application, Tokukaihei No. 9-68864 (Publication Date: Mar. 11, 1997) (U.S. Pat. No. 2,836,537), for example. In the developing apparatus, an insulating layer is laminated on a substrate made of metal or resin. In the insulating layer, plural sets of electrodes are disposed in series. Each set includes three electrodes. The electrodes generate an electric field curtain effect by using a progressive wave electric field. The electrodes function as transportation means. The transportation means constitutes a transportation path. Via the transportation path, developer is transported from a developing tank to a development position in proximity with an image supporting body, and transported back to the developing tank.
However, with the conventional progressive wave electric field method, the developing apparatus has the following problems. When electrically charged developer is transported to the transportation path, which is provided with the transportation means including the substrate and the insulating layer, secondary electric charge occurs between the developer and the insulating layer of the substrate. This changes a surface potential of the insulating layer. As a result, transportation of the developer is destabilized, for example, because the developer firmly adheres onto a surface of the transportation means. Moreover, when the surface potential of the insulating layer is changed, there is also a change in a developing potential, which is a potential difference between the image supporting body and the transportation means in a developing step. As a result, a stable state of development cannot be maintained.
The present invention also relates to a developing apparatus that develops, by using developer and the like, an electrostatic latent image formed on a latent image supporting body (image supporting body), and relates to an image forming apparatus including the developing apparatus. The present invention especially relates to a developing apparatus and an image forming apparatus that use a mechanism (electric field curtain) with which the developer is transported by using a progressive wave electric field.
The electrostatic latent image is not limited to an electrostatic latent image formed by writing optical information onto an image supporting body electrically charged with a predetermined charge. The electrostatic latent image may be formed, by a method such as an ion flow method, directly on a dielectric material. Alternatively, the electrostatic latent image may be directly formed by a method such as a toner jet method, in which (i) an arbitrary voltage is applied to an electrode having a plurality of openings, so as to form a latent image in the air, and (ii) the developer is sprayed on a recording medium.
Among developing apparatuses used in image forming apparatuses, such as copying apparatuses, printers, and facsimiles, in which photoelectric image forming is performed, attention is currently paid to such developing apparatuses that use a non-contact method. In the non-contact method, development is performed in such a manner that the developer supporting body contacts the image supporting body. Examples of the non-contact method are the powder cloud method, the jumping method, and the electric field curtain (progressive wave electric field) method.
As described in, for example, Japanese Publication for Unexamined Patent Application, Tokukaihei No. 9-68864, means for generating an electric field curtain includes (i) a supporting substrate made of metal or resin, and (ii) an insulating layer laminated on the supporting substrate. In the insulating layer, plural sets of electrodes are disposed in series. The electrodes generate an electric field curtain effect. Each set of the electrodes includes three electrodes. When multiphase voltages are applied to the electrodes, a progressive wave electric field is formed. By using the progressive wave electric field, developer is transported on a surface of developer transportation member.
Incidentally, for developing apparatuses using a progressive wave electric field, it is necessary to appropriately select (i) a pitch between adjacent ones of the electrodes of a developer transportation member and (ii) a driving frequency of the progressive wave electric field, so as to transport the developer efficiently and stably.
Specifically, if the pitch between adjacent ones of the electrodes is wide, a long time (transport time) is required for the developer to move between adjacent ones of the electrodes, although field intensity is high. It is therefore necessary to apply low-frequency voltages to the electrodes, so that an amount of the developer transported per unit time is maximized at a low frequency. On the other hand, if the pitch between adjacent ones of the electrodes is narrow, only a short time is required for the developer to move between adjacent ones of the electrodes, although the field intensity is low. It is therefore necessary to apply high-frequency voltages to the electrodes, so that the amount of the developer transported per unit time is maximized at a high frequency.
Thus, depending on whether the pitch between adjacent ones of the electrodes is wide or narrow, the amount of the developer transported per unit time significantly varies in a frequency band of the voltages applied to the electrodes. Therefore, unless an appropriate developer transportation condition is selected, it is impossible to transport the developer efficiently on the developer transportation means by using the progressive wave electric field.
Moreover, under the following conditions, developing apparatuses using a progressive wave electric field is susceptible to spatial and temporal distribution of a potential generated by the voltages applied to the electrodes: (i) a progressive wave electric field is generated on the surface (developer transportation surface) of the developer transportation member, that is, different voltages are applied to the electrodes, and (ii) a supporting surface (front surface) of the image supporting body, such as a photosensitive body, is right above or in close proximity with the surface of the developer transportation member after the image supporting body has moved in a perimeter direction. Therefore, when the supporting surface of the image supporting body is in close proximity with the surface of the developer transportation member, there is a possibility that periodical change in density is caused because of (i) the frequency of the voltages applied to the electrodes, (ii) the pitch between adjacent ones of the electrodes, and (3) a peripheral velocity of the image supporting body, as shown in FIGS. 26(a) and 26(b), when the electrostatic latent image on the supporting surface of the image supporting body is developed.
In order to transport the developer on the surface of the developer transportation member, the progressive wave electric field needs to have certain intensity. The field intensity is influenced by the pitch between adjacent ones of the electrodes, and by a potential difference between adjacent ones of the electrodes. This is because, in order to attain progressive wave electric field intensity necessary for transporting the developer, the potential difference between adjacent ones of the electrodes needs to be larger as the pitch between adjacent ones of the electrodes becomes wider. Here, a state of the developer transported by the progressive wave electric field is as follows. The developer is in a cloud-like state while being transported by the progressive wave electric field. When the developer is in the cloud-like state, a height of the developer from the surface of the developer transportation member becomes higher if the pitch between adjacent ones of the electrodes becomes wider. If the pitch between adjacent ones of the electrodes is wide, the potential difference between adjacent ones of the electrodes needs to be large, so as to attain a desired progressive wave electric field. Therefore, energy of movement (energy of movement qV where q is charge of the developer, and V is the potential difference) given to the developer becomes large. This causes collisions of the developer with the developer itself, and increases a transportation speed of the developer. In this way, the developer in the cloud-like state is laminated on the surface of the developer transportation member due to, for example, deflection of a spraying path caused by an influence of air resistance. As a result, the height of the developer tends to become higher. Therefore, if the supporting surface of the image supporting body is completely soaked into the developer that is in the cloud-like state, so-called blotching is caused. Blotching is a phenomenon in which the developer adheres to a non-developing area of the supporting surface. The non-developing area is an area to which adhesion of the developer is not intended.
As described above, in image forming apparatuses such as copying apparatuses and printers employing an electrophotographic method, developing apparatuses employing a non-contact method are often used. In the non-contact method, the developer is transported to a vicinity of the image supporting body, and the developer is sprayed on an electrostatic latent image on the image supporting body, so that the electrostatic latent image is developed. Examples of the non-contact method are the powder cloud method, the jumping method, and the electric field curtain (progressive wave electric field) method.
A method using a progressive wave electric field is described in Tokukaihei 9-68864, for example. In this method, there are provided (i) a transportation path through which developer is transported from a developer containing section to an image supporting body, (ii) a collecting path through which unnecessary developer that did not adhere to the image supporting body is collected, and (iii) a developing electrode, which is directed downward and which is provided in proximity with one end of the transportation path so as to face the image supporting body.
The transportation path has a large number of electrodes embedded therein. To the electrodes, multiphase alternating voltages are applied, so as to generate a progressive wave electric field. By using the progressive wave electric field, the developer on the transportation path is transported to the image supporting body. When the developer is transported to the vicinity of the image supporting body, the developer is sprayed on the image supporting body due to charge of the electrostatic latent image of the image supporting body and due to the field generated by the developing electrode. Then, the developer adheres to the electrostatic latent image. In this way, the electrostatic latent image on the image supporting body is developed. The developer that did not adhere to the electrostatic latent image falls into the collecting path, and is collected through the collecting path into the developer containing section.
In such a developing apparatus, no mechanical power is used for transporting the developer. Instead, only the multiphase alternating voltages are applied to the electrodes of the transportation path. Therefore, it is possible to simplify an arrangement of the apparatus, and to miniaturize the apparatus.
Incidentally, in a method using a progressive wave electric field, distribution of toner in the transportation path is uneven in a traveling direction. Therefore, periodical density unevenness of the toner is caused. This is due to a frequency of the multiphase alternating current applied to the electrodes of the transportation path. If a toner image is formed by adhering the toner to the electrostatic latent image on the image supporting body while the uneven density of the toner on the transportation path moves in parallel with the electrostatic latent image in such a manner as to face the electrostatic latent image, the uneven density of the toner is directly reflected in the toner image.
However, in the foregoing conventional developing apparatus, despite the fact that (i) a traveling direction of the developer is a rotation movement direction of the image supporting body, and (ii) the toner having the uneven density on the transportation path faces the electrostatic latent image on the image supporting body and moves in parallel with the electrostatic latent image, no measure has been taken to suppress an influence of the density unevenness of the toner.